In situ UV-vis studies of the effect of particle size on the epoxidation of ethylene and propylene on supported silver catalysts with molecular oxygen

In this study the effect of particle size on ethylene and propylene epoxidation was studied on a series of silver catalysts supported on CaCO ₃ with loading levels of 0.5-56 wt%. Particle sizes determined from O₂ chemisorption uptakes at 443 K (170 °C) ranged from 50 to 660 nm, and this was confirmed by field emission scanning electron microscopy, which in addition showed crystallite agglomeration at high loadings. Reaction results show that large particles favor ethylene epoxidation by 3-5-fold at 473-493 K, whereas particle size does not have a large effect on propylene epoxidation. X-Ray diffraction measurements indicate that the bulk of the particles consist of silver in a metallic state, but in situ ultraviolet-visible (UV-vis) spectroscopy distinctly shows that in addition to a metallic component, small particles have silver in Ag⁺ state. The small particles are probably covered by a layer of Ag₂O, which results in lower selectivity for epoxidation for both propylene and ethylene oxidation in the small size regime. The approach to steady state is fast in propylene oxidation (3-4 h) and is accompanied by changes in the UV-vis spectra that indicate a reduction in the Ag₂O phase. The approach to steady state is slow in ethylene oxidation (24-36 h) and is associated with changes in the UV-vis spectra that are consistent with the formation of a partially oxidized surface phase, which may involve subsurface or adsorbed oxygen.